Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system or an electrostatic recording system.
Description of the Related Art
Hitherto, as an image forming apparatus using an electrophotographic system or an electrostatic recording system, there exists an image forming apparatus using an intermediate transfer system. In the intermediate transfer system, toner images formed on a plurality of image bearing members are primarily transferred onto an intermediate transferring member so as to be overlapped with each other. Thereafter, the toner images are secondarily transferred from the intermediate transferring member onto a recording material, for example, paper, thereby outputting an image.
In the image forming apparatus described above, a toner (secondary transfer residual toner) remains on the intermediate transferring member after the secondary transfer step. Therefore, the secondary transfer residual toner is required to be removed from the intermediate transferring member so as to be collected. As a method of removing the secondary transfer residual toner from the intermediate transferring member, the following method is known. Specifically, after the secondary transfer residual toner is charged to a polarity opposite to an original charging polarity of the toner by a charging unit, the secondary transfer residual toner is transferred onto a photosensitive member simultaneously with the primary transfer at a primary transfer portion so as to be collected by a cleaning unit of the photosensitive member (Japanese Patent No. 3267507).
However, it is found that a configuration for transferring the secondary transfer residual toner onto the photosensitive member and collecting the secondary transfer residual toner after charging the secondary transfer residual toner to the polarity opposite to the original charging polarity of the toner has the following problem.
When the secondary transfer residual toner is strongly charged to the polarity opposite to the original charging polarity of the toner, the secondary transfer residual toner is collected by the photosensitive member at the primary transfer portion after being further strongly charged by the charging unit. At this time, the secondary transfer residual toner is collected by the photosensitive member together with (while electrostatically adsorbing) the toner with the original charging polarity that is to be primarily transferred onto the intermediate transferring member at the primary transfer portion. As a result, an image defect called “ghost” disadvantageously occurs. A mechanism of causing the ghost is now described.
Charges of the secondary transfer residual toner greatly change depending on a voltage applied to a secondary transfer portion. When the voltage applied to the secondary transfer portion is low, an electric field for transferring the toner from the intermediate transfer member onto the recording material is weak. Therefore, the toner having the charges with the original charging polarity remains on the intermediate transferring member as the secondary transfer residual toner without being successfully transferred onto the recording material (this phenomenon is hereinafter referred to as “weak hollow”). On the other hand, when the voltage applied to the secondary transfer portion is high, a discharge occurs between the intermediate transferring member and the recording material. As a result, the toner has charges with the polarity opposite to the original charging polarity, and therefore remains on the intermediate transferring member as the secondary transfer residual toner without being transferred onto the recording material (this phenomenon is hereinafter referred to as “strong hollow”). Therefore, it is desirable to apply to the secondary transfer portion an appropriate voltage that does not cause the “weak hollow” or the “strong hollow”.
Meanwhile, when the voltage applied to the secondary transfer portion is lowered to such a level that the “strong hollow” does not occur, an image defect called “explosion” sometimes occurs independently of the above-mentioned “weak hollow”. The “explosion” is an image defect caused in the following manner. Specifically, when charges on the recording material charged at the secondary transfer portion are low, the toner transferred onto the recording material cannot be sufficiently attracted to the recording material by an electrical force. As a result, the toner is scattered onto a non-image portion of the recording material to cause an image defect appearing as if the toner exploded. The “explosion” is liable to occur when an electric resistance of the recording material is high or an electric resistance of a secondary transfer roller used to apply the voltage to the secondary transfer portion is low.
Here, the “strong hollow” is an image defect appearing as a slightly low image-density portion which cannot be visually observed on an image or as extremely small sparsely scattered dot-like white spots. In contrast, the “explosion” appears as an explosion-like pattern on the image. Therefore, the “explosion” is more noticeable than the “strong hollow” as the image defect.
Thus, suppression of the “explosion” is sometimes regarded as priority by slightly increasing the voltage applied to the secondary transfer portion. When the voltage that may cause the “strong hollow” is applied to the secondary transfer portion, the secondary transfer residual toner has strong charges with the polarity opposite to the original charging polarity. As a result, the amount of charges is further increased by the charging unit, thereby sometimes causing the above-mentioned “ghost”.